DS/CDMA receiver using parallel-operating multi-purpose correlators

ABSTRACT

In a direct sequence spread spectrum receiver, multiple correlators are provided for despreading a spread data signal and a spread pilot signal. During a sync acquisition mode, pilot demodulators are connected to the correlators, respectively, and during a subsequent tracking mode, data demodulators are connected to the correlators whose outputs are combined by an adder to produce an output for the receiver. Despreading codes of different phase positions are generated and supplied to the correlators, respectively. During the sync acquisition mode, the phase position of each despreading code is successively shifted and high correlation values are determined from the outputs of the pilot demodulators. During the tracking mode, the phase positions of the despreading codes are set according to the high correlation values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to direct sequence spread spectrum (code division multiple access) receivers. This invention is particularly concerned with a sync acquisition and tracking technique for direct sequence spread spectrum receivers of cellular mobile communications systems in which transmitted signals are severely affected by Rayleigh fading and multipath fading.

2. Description of the Related Art

Commercial interest in direct sequence spread spectrum (code division multiple access) communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the cell-site station of the DS/CDMA system, a data symbol is spread by multiplying it with orthogonal pseudo-random number (PN) sequences assigned to the cell site as well as with orthogonal Walsh codes assigned to the channel over which the spread signal is transmitted. In order to enable the mobile station to provide sync acquisition and tracking operation, a pilot signal is superimposed on the data symbol sequence. At the mobile station, a sliding correlation technique is used to shift the phase timing of a local PN sequence by a predetermined amount each time a correlation is taken between the received and local sequences and determine the correct phase timing for the local sequence when the correlation exceeds some critical value. Such phase shifting is performed at predetermined "window" intervals. Once synchronization is established, the phase difference is monitored and maintained to within one half of the chip interval. During transmission, the signal undergoes reflections from various land structures, producing a complex pattern of standing waves due to mutual interference. As a result, the propagation path of the signal exhibits a field intensity distribution which is approximated by the Rayleigh distribution. Thus, the signal experiences a phenomenon called "Rayleigh fading" and the envelope of the signal at the mobile station as well as its phase violently fluctuate.

Under such unfavorable conditions, the sliding correlation technique is not ideal to achieve quick synchronization in response to the rapidly varying signal levels. In addition, there is often a need to update the mobile's phase timing due to the arrival of a strong reflection or a signal of significant level from an adjacent cell site. Under such circumstances, the prior art system sets the correlation circuitry to the new phase timing immediately in response to the arrival of a new strong signal without taking the old symbols, which may be left in the correlation circuitry, into account. The use of the old symbols for the new phase timing results in timing discontinuity and produces over-correlation or under-correlation.

SUMMARY OF THE INVENTION

The primary object of the present invention is therefore to provide a direct sequence spread spectrum receiver capable of establishing quick synchronization under varying signal levels.

Another object of the present invention is to provide a direct sequence spread spectrum receiver which ensures seamless transition when updating the phase timing of correlation process.

According to the present invention, there is provided a direct sequence spread spectrum receiver which comprises a plurality of correlators for despreading a spread data signal and a spread pilot signal, a plurality of pilot demodulators, a plurality of data demodulators and an adder. The data demodulators and the adder constitute a RAKE receiver. During a sync acquisition mode, the pilot demodulators are connected to the correlators, respectively, and during a subsequent tracking mode, the data demodulators are connected to the correlators to produce a plurality of demodulated data signals which are combined by the adder to produce an output for the RAKE receiver. A plurality of despreading codes of different phase positions are generated and supplied to the correlators, respectively, causing the correlators to despread one of the spread data and pilot signals. During the sync acquisition mode, the phase position of each despreading code is successively shifted and high correlation values are determined from the outputs of the pilot demodulators. During the tracking mode, the phase positions of the despreading codes are set according to the high correlation values. Since multiple correlators operate in parallel during the sync acquisition mode, the time taken to search through the range of possible phase positions is significantly reduced. In practical aspect, the data demodulators are provided in number corresponding to the number of multipath fading channels.

The despreading codes are generated such that, during the sync acquisition mode, all the correlators provide correlations for the incoming pilot signal only, and during the tracking mode, one half of the correlators provide correlation for the data signal and the remainder provides correlation for the pilot signal.

The despreading code comprises a pseudo-random number (noise) sequence uniquely identifying a cell site area and a Walsh code identifying a communication channel. When the Walsh code is set to an all-zero code, the correlators supplied with this code is set to provide correlation with the pilot signal.

In a preferred aspect, a second despreading code of different phase position is supplied to a specified one of the correlators during the tracking mode to continue searching for best phase positions. The phase position of the second despreading code is shifted in the neighborhood of those previously set in the sync acquisition mode and high correlation values are newly determined from output signals from one of the pilot demodulators. The previously set phase positions are updated according to the new high correlation values during an update mode.

To ensure smooth transition when new phase positions are determined and old phase positions are to be updated, a second data demodulator is provided. This data demodulator is connected to the output of the specified correlator at the time the specified correlator is set to a new phase position. In response to the connection of the second data demodulator to the specified correlator, a ramp-up time is introduced to allow old symbols to be replaced with new symbols, and immediately following the ramp-up time, a successive one of the outputs of the other data demodulators is disconnected from the adder for a predetermined interval and, instead, the output of the second data demodulator is connected to the adder during that predetermined interval.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a direct sequence spread spectrum receiver according to the present invention;

FIG. 2 is a circuit diagram of each correlator of FIG. 1;

FIGS. 3A, 3B and 3C are block diagrams of the processor FIG. 1 during sync acquisition, tracking and update modes, respectively;

FIG. 4 is a circuit diagram of each pilot demodulator of FIGS. 3A, 3B and 3C;

FIGS. 5A and 5B are circuit diagrams of different embodiments of each data demodulator of FIGS. 3A, 3B and 3C; and

FIGS. 6A, 6B and 6C are block diagrams of the phase shift controller FIG. 1 during sync acquisition, tracking and update modes, respectively.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown a receiving circuit of a mobile station for a DS/CDMA cellular communications system according to the present invention. At a cell-site station, a baseband downlink (cell-to-mobile) signal is initially encoded by a channel encoder into a known coded form that is optimized for radio transmission. At the chip rate much higher than the symbol rate, the symbols of the encoded signal are spread with PN (pseudo-random number) spreading sequences PN_(i) and PN_(q) commonly assigned to the service area in which the mobile station is currently located and further spread with orthogonal Walsh codes uniquely assigned to the downlink channel. A pilot signal, which may be a series of all zeros or ones, is spread at the same chip rate with the same PN sequences PN_(i) and PN_(q) with orthogonal Walsh all-zero codes. The in-phase and quadrature components of spread spectrum downlink signal are combined with the corresponding components of spread spectrum pilot signal to produce an I signal and a Q signal which are modulated onto orthogonal radio-frequency carriers, amplified and transmitted.

The spread spectrum signal from the cell-site is received by the mobile station and supplied to a quadrature demodulator 10 which includes radio-frequency amplification and demodulation stages. Using orthogonal local carriers, the demodulator 10 recovers the original i and q signals. After analog-to-digital conversion by an A/D converter 11, the digital i and q signals are fed in parallel to correlators CR₁ to CR₈ of identical configuration.

As illustrated in FIG. 2, each of the correlators CR_(k) (where k=1, 2, . . . ,8) comprises a pair of multipliers 20i and 20q for multiplying the I and Q digital signals from A/D converter 11 with the pseudo-random despreading sequences PN_(i)(k) and PN_(q)(k) to produce an in-phase output i_(k) =I×PN_(ik) from multiplier 20i and a quadrature output signal q_(k) =Q×PN_(qk) from multiplier 20q. The PN sequences PN_(ik) and PN_(qk) are the two inputs to an exclusive-OR gate 30 producing a switching signal. The output signal i_(k) of multiplier 20i and the output signal q_(k) of multiplier 20q are the inputs to switches 21i and 21q. If the switching signal is a binary 1, the output of multiplier 21i appears at the output of switch 21i as a signal I_(dk). If the switching signal is a binary 0, the signal I_(dk) is equal to the output of multiplier 21q. Similarly, the output of multiplier 20q appears at the output of switch 21q as a signal Q_(dk) when the switching signal is 1. Otherwise, the output of multiplier 20i appears as Q_(dk). Correlator CR_(k) further includes a pair of multipliers 23i and 23q for despreading the signals I_(dk) and Q_(dk) with a Walsh code W_(k). As will be described, all the Walsh codes supplied to correlators CR₁ to CR₈ are all-zero Walsh codes when the receiver is in a sync acquisition mode and some of these codes change to distinct Walsh codes of different Walsh code numbers when it is in a tracking mode. Since the pilot signal was spread at the transmitter with the all-zero Walsh code, the outputs of multipliers 23i and 23q represent the in-phase and quadrature components of the despread pilot signal during the sync acquisition mode and represent the in-phase and quadrature components of the despread data signal during the tracking mode. The outputs of multipliers 23i, 23q are respectively integrated over a symbol interval to produce in-phase and quadrature pilot or data symbols I_(k), Q_(k).

Returning to FIG. 1, the orthogonal output signals from each of the correlators CR are supplied to a data processor 12 where they are processed in a manner to be described and applied to a phase shift controller 13 as correlation sum signals S1˜S8. During the sync acquisition mode, pilot symbols are demodulated from the outputs of all correlators and applied to the phase shift controller. During the tracking mode, data symbols are demodulated from the outputs of correlators CR₁ to CR₆ and a sum of these signals is supplied to a channel decoder 14, while pilot signals are demodulated from the outputs of correlators CR₇ and CR₈ and applied to the phase shift controller as signals S7 and S8 and binary 0's are fed to the phase shift controller as signals S1 to S6. The CDMA receiver enters an update mode in which the phase timing obtained during the tracking mode is used to update the phase timing of the PN sequences applied the correlators CR₇ and CR₈ which were used during the tracking mode to produce pilot symbols. In this update mode, the outputs of all correlators are used to recover data symbols which are summed together and applied to the channel decoder 14.

In response to signals S1˜S8, the phase shift controller 13 produces phase position signals P1˜P8 and Walsh code numbers N1˜N8. During the sync acquisition mode, the phase shift controller produces eight sequences of different phase position signals and supplies these sequences simultaneously as sequences P1˜P8 to PN sequence generators PG₁ ˜PG₈ and Walsh code generators WG₁ ˜WG₈, respectively, and supplies zero Walsh code numbers (ω₀) to all the Walsh code generators. During this acquisition mode, the phase shift controller makes a search through the correlation sum signals S1 to S8 generated as a result of successive phase shifts through all the range of the PN sequence (corresponding to the shifts of 0 to 2¹⁶ bits) and determines the highest three values and identifies corresponding phase positions Δθ₁ to Δθ₃.

During the tracking mode, these phase position signals are applied as signals P1 to P6 to PN generators PG₁ to PG₆ in pairs so that the PN generators of each pair are driven by the same phase position. PN generators PG₇ and PG₈ are then sequentially driven by signals P7 and P8 at phase positions which are located in the neighborhood of Δθ₁ to Δθ₃ to continue the search to precisely determine the optimum phase positions from the signals P7 and P8. At the same time, Walsh code numbers ω₁, ω₃ and ω₅ are respectively supplied to Walsh code generators WG₁, WG₃ and WG₅ corresponding to phase positions Δθ₁, Δθ₂ and Δθ₃, respectively, and zero code numbers Δθ₀ are supplied to Walsh code generators WG₂, WG₄, and WG₆ to WG₈. As a result, during the tracking mode, the outputs of correlators CR_(k) (where k=1, 3 and 4) are orthogonal data symbols, while the outputs of correlators CR_(k+1) are orthogonal pilot symbols.

During the update mode, the phase position signals Δθ₁ to Δθ₃ are respectively updated with the optimum phase positions represented by signals P7 and P8.

By using the phase position signals P1˜P8, PN generators PG₁ ˜PG₈ respectively produce pairs of PN sequences PN_(i1), PN_(q1), PN_(i2), PN_(q2), . . . , PN_(i8), PN_(q8) for coupling to correlators CR₁ ˜CR₈, respectively. Each Walsh code generator WG_(k) uses a phase position signal P(k) and a corresponding code number N(k) to produce a Walsh code at the phase timing of the signal P(k) for coupling to the corresponding correlator CR_(k).

Details of the data processor 12 are shown in FIGS. 3A, 3B and 3C for sync acquisition, tracking and update modes, respectively. As illustrated in FIG. 3A, the processor 12 includes three groups of switches 31-i, 32-i and 33-j (where i=1˜4, j=1˜3). All of these switches operate in response to a switching signal supplied from a mode controller 22. The outputs of correlators CR₁ to CR₈ are coupled in pairs via corresponding switches 31-1, 31-2, 31-3 and 31-4 to pairs of pilot demodulators (PD₁, PD₂), (PD₃, PD₄),(PD₅, PD₆) and (PD₇, PD₈) or data demodulators DD₁, DD₂, DD₃ and DD₄. The outputs of pilot demodulators of each pair are supplied via switch 32 corresponding to that pair to phase shift controller 13 as correlation sum signals S1 and S2 and the output of the data demodulator associated with that pair is supplied via corresponding switch 33 to an adder 34 as a signal "r_(i) ". Therefore, the outputs of correlators CR.sub. 3 and CR₄, for example, are coupled via switch 31-2 to pilot demodulators PD₃, PD₄ or data demodulator DD₂ and the outputs of these pilot demodulators are coupled via switch 32-2 to phase shift controller 13 as correlation sum signals S3 and S4 and the output of data demodulator DD₂ being coupled via switch 33-2 to adder 34 as a signal r₂.

Mode controller 30 supplies a mode indicating signal to phase shift controller 13 and responds to a signal from phase shift controller 13 by operating the switches to change the operating mode to the next.

During the sync acquisition mode, mode controller 30 set all the switches so that the orthogonal output signals I_(k) and Q_(k) (where k=1, 2, . . . , 8) are coupled to corresponding pilot demodulator PD_(k) to supply correlation sum signals S1 to S8 to phase shift controller 13, and all the inputs to adder 34 are set to zero.

During the tracking mode (FIG. 3B), the outputs of correlators CR₁ to CR₆ are switched to the corresponding data demodulators DD₁ ˜DD₃ producing output signals r₁, r₂ r₃, which are connected via switches 33-1, 33-2, 33-3 to the adder 34. Switches 32-1, 32-2 and 32-3 are switched to the binary 0 position to set the output level of signals S1 to S6 to zero. On the other hand, the outputs of correlators CR₇ and CR₈ remain connected to pilot demodulators PD₇ and PD₈ to produce correlation sum signals S7 and S8.

During the update mode (FIG. 3C), the outputs of all the correlators are connected in pairs to the corresponding data demodulators to generate output signals r₁ ˜r₄. As will be described, switches 33-1, 33-2 and 33-3 are controlled to combine two of the signals r₁, r₂ and r₃ with the signal r₄ by adder 34 for a brief period immediately following a "ramp-up" time.

As shown in FIG. 4, each pilot demodulator includes a pair of integrators 40i and 40q, a pair of squaring circuits 41i and 41q and an adder 42. The in-phase and quadrature signals I_(k) and Q_(k) from correlator CR_(k) are respectively summed by integrators 40i, 40q during the period of "m" symbols to produce orthogonal sum signals which are squared respectively by the squaring circuits 41i and 41q. The sum-and-squared signals are combined together by adder 42 to produce the correlation sum signal S_(k) for the correlator CR_(k).

Details of each data demodulator DD_(j) according to one embodiment of this invention are illustrated in FIG. 5A (where j=1, 2, . . . 4). Each data demodulator DD_(j) includes a pair of multipliers 43i and 43q and an adder 44. Multiplier 43i takes correlation between the output signal I_(k) of correlator CR_(k) and the output signal I_(k+1) of correlator CR_(k+1) and multiplier 43q takes correlation between the output signals Q_(k) and Q_(k+1). As described above, during the tracking mode, output signals I_(k) and Q_(k) are orthogonal data symbols, while output signals I_(k+1) and Q_(k+1) are orthogonal pilot symbols. Therefore, the multiplication of a data symbol of each in-phase or quadrature component with a pilot symbol of the corresponding orthogonal component at multiplier 43 causes the data symbol to be rotated to the reference axis of the I-Q plane and weighted by the amplitude of the pilot symbol. The outputs of multipliers 43i and 43q are summed together by adder 44 to produce an output signal r_(j). The output of adder 44 is supplied to the channel decoder 14 where it undergoes a process inverse to that of the transmitter's channel encoder.

The amplitude of the pilot signal is usually much higher than that of the data signal, and the phase difference between these signals is negligibly small. In order to obtain the output signal r_(j) the data signal vector is projected onto the I axis of the I-Q complex plane by multiplying each data signal component with a complex conjugate of the corresponding pilot signal component and summing the products as follows: ##EQU1## where r_(d) is the amplitude of the data symbol, r_(p) is the amplitude of the pilot symbol, θ_(d) and θ_(p) are the angle of phase of the data and pilot symbols, respectively. Equation (1) indicates that a unit vector of the data signal is rotated clockwise to the I axis of the I-Q complex plane and weighted by the scalar product r_(p) r_(d).

Rotation of the data vector onto the I-axis of the I-Q complex plane allows for the simple summation of signals at the inputs of the adder 34 of processor 12. The use of the pilot vector in the rotation method described above reduces the effect of fading on the signal outputted from adder 24 as the pilot vector weights the data symbol and stronger multipath signals will have stronger moving-averaged pilot strong signals which indicate low levels of fading.

Another, but preferred embodiment of each data demodulator is shown in FIG. 5B in which each data demodulator further includes a pair of delay circuits 45i, 45q for introducing a delay of one half of "s" data symbols to the output signals I_(k) and Q_(k), respectively, and a pair of moving average circuits 46i, 46q for successively integrating "s" symbols of the output signals I_(k+1) and Q_(k+1), respectively, to produce a moving average value of pilot signals. Each of the moving average circuits is implemented with an s-stage shift register and an adder. The shift register receives an input signal from the preceding circuit and the adder is connected to all the stages of the register for successively summing the values of their contents so that the adder produces a signal representing the moving average of the values of "s" pilot symbols (corresponding to "s" data symbols) for each phase component. Thus, for each delayed data symbol, a moving average value is derived from s/2 pilot symbols that precede the delayed symbol as well as from s/2 pilot symbols that succeed it. If several data symbols are severely corrupted by noise during transmission, the corresponding pilot symbols are moving-averaged and the effect of the noise on the pilot signal is reduced in this manner, leading to a reduction in the effect of the noise on the signal r_(j).

The outputs of delay circuit 45i and 45q are supplied respectively to the multipliers 43i and 43q and the moving average values of in-phase and quadrature components are supplied respectively to multipliers 43i and 43q. Since the incoming data symbols are always subjected to multipath fading and Rayleigh fading, an incoming symbol may be corrupted by random noise during the time the corresponding pilot symbol is moving-averaged over the period of "s" symbols. The introduction of s/2-symbol delays has the effect of reducing noise-related problems to a minimum.

Referring to FIGS. 6A to 6C, details of the phase shift controller 13 are illustrated for the sync acquisition, tracking and update modes of operation, respectively. The phase shift controller comprises a controller 60, a phase position data source 61, a selector 62 and a Walsh code number generator 63.

In FIG. 6A, controller 60 is responsive to a mode indicating signal from processor 12. During a sync acquisition mode, controller 60 commands the phase position data source 61 to simultaneously generate phase position sequences Pk (where k=1, 2, . . . , 8) each comprising position signals Δθ_(k1) to Δθ_(kn). The phase position signals P1˜P8 are applied through selector 62 to the PN generators PG₁ ˜PG₈ and Walsh code generators WG₁ ˜WG₈, respectively. At the same time, controller 60 commands the Walsh code number generator 63 to generate zero Walsh code numbers ω₀ and applies these numbers as signals N1˜N8 to the Walsh code generators WG₁ ˜WG₈, respectively.

In response to a sequence P(k) of phase position signals Δθ_(k1) to Δθ_(kn), each PN generator PN_(k) (FIG. 1) successively supplies a sequence of in-phase pseudo-random numbers PN_(i)(k) and a sequence of quadrature pseudo-random numbers PN_(q)(k) to the corresponding correlator CR_(k). At the same time, the correlator CR_(k) is supplied with a Walsh code sequence W_(k) that is produced by the Walsh code generator WG_(k) according to the corresponding Walsh code number N(k) at successive phase positions Δθ_(k1) to Δθ_(kn).

All the correlators CR operate in parallel to take correlations between the incoming signal and the respective despreading codes (each comprising PN sequences and a Walsh sequence) during the sync acquisition mode, using divided sequences of phase position signals to all phase shift controller 13. Since the Walsh code sequences are all-zero sequences during the sync acquisition mode, mode controller 30 at data processor 12 initially sets all the switches A, B, C, D in a manner as illustrated in FIG. 3A, so that all pilot demodulators PD₁ ˜PD₈ are connected to the outputs of corresponding correlators CR₁ ˜CR₈ to produce correlation sum signals S1 through S8.

Returning to FIG. 6A, controller 60 is responsive to the signals S1˜S8 for selecting the highest three values of correlation sums from all phase positions Δθ₁₁ to Δθ_(8n) and the first, second and third highest values are stored into registers RS1, RS2 and RS3, respectively. Phase position signals Δθ₁, Δθ₂ and Δθ₃ corresponding respectively to the first, second and third highest values are detected by controller 60 and stored into registers RP1, RP2 and RP3, respectively. Controller 60 returns an end-of-search signal signifying the end of a sync acquisition mode to the processor 12 to allow it to proceed to a tracking mode. As a result, the amount of time taken to complete a search through the whole range of phase positions is therefore 1/8 of the sync acquisition time which would be taken by a single correlator using a non-divided sequence of phase positions Δθ₁₁ to Δθ_(8n).

In response to the end-of-search signal, the data processor 12 operates the switches 31, 32 and 33 as shown in FIG. 3B for coupling the inputs of data demodulators DD₁ to DD₃ to the outputs of corresponding correlators CR₁ to CR₆ and their outputs to adder 34 and disconnecting pilot demodulators PD₁ to PD₃ from their associated correlators CR to set the level of signals S1 to S6 to zero. The outputs of pilot demodulators PD₇ and PD₈ continue producing the signals S7 and S8. Mode controller 30 supplies a tracking mode signal to the controller 60 of phase shift controller 13.

In FIG. 6B, controller 60 responds to the tracking mode signal by controlling the selector 62 to read the phase position signals stored in registers RP1 to RP3 and delivered from the selector as signals P1 to P6. Specifically, the first phase position signal Δθ₁ in register RP1 is coupled to PN generators PG1 and PG2 as signals P1 and P2, the second phase position signal Δθ₂ in register RP2 being coupled to PN generators PG3 and PG4 as signals P3 and P4, and the phase position signal Δθ₃ in register RP3 being coupled to PN generators PG5 and PG6 as signals P5 and P6. Concurrently, controller 60 selects phase positions (Δθ₁₁ ' to Δθ_(1m) ') and (Δθ₁₁ " to Δθ_(1m) ") in the neighborhood of the phase position Δθ₁, phase positions (Δθ₂₁ ' to Δθ_(2m) ') and (Δθ₂₁ " to Δθ_(2m) ") in the neighborhood of the phase position Δ θ₂, and phase positions (Δθ₃₁ ' to Δθ_(3m) ') and (Δθ₃₁ " to Δθ_(3m) ") in the neighborhood of the phase position Δθ₃. Controller 60 directs the phase position data source 61 to simultaneously generate a first sequence of phase position signals (Δθ₂₁ ' to Δθ_(2m) '), (Δθ₂₁ ' to Δθ_(2m) ') and (Δθ₃₁ ' to Δθ_(3m) ') and a second sequence of phase position signals (Δθ₁₁ " to Δθ_(1m) "), (Δθ₂₁ " to Δθ_(2m) ") and (Δθ₃₁ " to Δθ_(3m) ") and supply the first and second sequences as signals P6 and P7 to the PN generators PG₇ and PG₈, respectively.

Controller 60 commands the Walsh code number generator 63 to supply the following code numbers:

N1=ω₁ (corresponding to Δθ₁)

N2=ω₀

N3=ω₃ (corresponding to Δθ₂)

N4=ω₀

N5=ω₅ (corresponding to Δθ₃)

N6=ω₀

N7=ω₀

N8=ω₀

As a result, correlators CR₁, CR₃, CR₅ produce orthogonal data signals (I₁, Q₁), (I₃, Q₃), (I₅, Q₅), respectively, while correlators CR₂, CR₄, CR₆ produce orthogonal pilot signals (I₂, Q₂), (I₄, Q₄), (I₆, Q₆), respectively. It is seen in FIG. 5A (or 5B), data signals I₁ and Q₁ are multiplied with pilot signal I₂ and Q₂ to produce an output signal r₁. Similarly, data signals I₃ and Q₃ are multiplied with pilot signal I₄ and Q₄, producing a signal r₂ and data signals I₅ and Q₅ are multiplied with pilot signal I₆ and Q₆, producing a signal r₃. The signals r₁, r₂ and r₃ are summed by adder 34 and fed to the channel decoder 14.

On the other hand, correlators CR₇ and CR₈ use the despreading sequences successively generated as a result of new phase positions to cause processor 12 to supply correlation sum signals S7 and S8 to controller 60 to allow it to search for the highest value of .these correlation sums during this tracking mode. The first, second and third highest values are registers RS1, RS2 and RS3, respectively, and corresponding phase position signals Δθ_(u1), Δθ_(u2) and Δθ_(u3) are stored into register RP4, RP5 and RP6, respectively. Controller 60 then supplies a start-of-update signal to the processor 12.

In response to the start-of-update signal, the phase shift controller 13 enters an update mode. To ensure seamless transition of phase positions from those detected in the sync acquisition mode to those detected in the tracking mode (without over-correlation or under-correlation), mode controller 30 (FIG. 3C) introduces a "ramp-up" time at the instant the PN generators PG₇ and PG₈ are set to a new phase position according to each of the new phase position signals Δθ_(u1), Δθ_(u2) and Δθ_(u3).

More specifically, mode controller 30 receives the start-of-update signal and directs the switch 31-4 to couple the outputs of correlators CR₇ and CR₈ to data demodulator DD₄ and introduces a ramp-up time (RUT) in response to the data demodulator DD₄ being coupled to the correlators CR₇, CR₈. If the circuit of FIG. 5A is used for the data demodulators, the ramp-up time corresponds to one symbol interval and the transient period (i.e., t₂ -t₃, t₄ -t₅, t₆ -t₇) in which the signal r₄ is connected to the adder also corresponds to one symbol interval. If use is made of the circuit of FIG. 5B both of the ramp-up time and the transient period correspond to the "s" symbol interval.

As shown in FIG. 6C, new phase position signals Δθ_(u1), Δθ_(u2) and Δθ_(u3) are respectively set into registers RP4, RP5 and RP6 and the selector 62 sets the phase position signals P7 and P8 to the new phase position Δθ_(u1) at time t₁ and Walsh code number generator 63 sets the code number N7 to ω_(u1) corresponding to Δθ_(u1).

In response to the start-of-update signal, mode controller 30 (FIG. 3C) introduces the ramp-up time corresponding to several symbols ("s" symbols in the case of FIG. 5B) which prevails until time t₂. The introduction of the ramp-up time allows previous residual data which may be left in correlators CR₇ and CR₈ and data demodulator DD₄ to be cleared and replaced with symbol or symbols resulting from the new phase position. Thus, the previous phase position Δθ₁ is still used and adder 34 produces a signal R=r₁ +r₂ +r₃ during the time prior to time t₂.

During the interval t₂ to t₃, mode controller 30 (FIG. 3C) controls the switch 33-1 to connect the output signal r₄ of demodulator DD₄ to adder 34, instead of signal r₁, producing a signal r₄ +r₂ +r₃.

At time t₃, register RP1 (FIG. 6C) is updated with the new phase position signal Δθ_(u1) so that PN generators PG₁ and PG₂ are thereafter driven at the new phase position Δθ_(u1) and Walsh code number generator 63 updates the code number N1 with ω_(u1) replacing the previous number ω₁ and sets the code number N7 to ω_(u3) corresponding to Δθ_(u2). Switch 33-1 (FIG. 3C) is returned to the uppermost position for coupling the output r₁ of demodulator DD₁ so that the output signal R of adder 34 during the interval t₃ to t₄ is equal to R=r₁ +r₂ +r₃ (see FIG. 6C).

Although the new phase position is used for the correlators CR₁ and CR₂, the signal r₂ resulting from the previous phase position Δθ₂ is still used during the interval t₃ to t₄. Therefore, the interval t₃ to t₄ is the ramp-up time for correlators CR₇ and CR₈ and demodulator DD₄ to allow their residual signals resulting from the phase position Δθ_(u1) to be cleared. During the subsequent interval t₄ to t₅, switch 33-2 is moved to the lowermost position for coupling the signal r₄, instead of signal r₂, to adder 34, producing a signal r₁ +r₄ +r₃. At time t₅, register RP2 (FIG. 6C) is updated with the new phase position signal Δθ_(u2) so that PN generators PG₃ and PG₄ are thereafter driven at the new phase position Δθ_(u2). Concurrently, Walsh code number generator 63 updates the code number N3 by replacing ω ₃ with ω_(u3) corresponding to the new phase position Δθ_(u2).

During the subsequent interval t₅ to t₆, a ramp-up time is introduced for correlators CR₇, CR₈ and demodulator DD₄ to allow their residual signals resulting from the phase position signal Δθ_(u2) to be cleared in preparation for the new phase position Δθ_(u3). During interval t₆ to t₇, switch 33-3 is moved to the lowermost position for coupling the signal r₄ to adder 34, instead of signal r₃, producing a signal r₁ +r₂ +r₄. At time t₇, register RP3 (FIG. 6C) is updated with the new phase position signal Δθ_(u3) so that PN generators PG₅ and PG₆ are thereafter driven at the new phase position Δθ_(u3). Concurrently, Walsh code number generator 63 updates the code number N5 by replacing ω₅ with ω_(u5) corresponding to the new phase position Δθ_(u3).

At time t₇, controller 60 controller 60 (FIG. 6B) directs the phase position data source 61 to regenerate the previously mentioned first and second sequences of phase position signals corresponding to those in the neighborhood of phase positions Δθ₁, Δθ₂ and Δθ₃ and apply these sequences as phase position signals P7 and P8 to PN generators PG₇ and PG₈ to resume the search for optimum phase positions and commands the Walsh code number generator 63 to set the code number N7 to ω₀. At the same time, controller 60 supplies an end-of-update signal to processor 12. In response, mode controller 30 of the processor operates switch 33-3 to return to the uppermost position, producing a signal r₁ +r₂ +r₃. The receiver now resumes the tracking mode. 

What is claimed is:
 1. A direct sequence spread spectrum receiver comprising:a plurality of correlators for despreading a spread data signal and a spread pilot signal; a plurality of pilot demodulators; a plurality of data demodulators; an adder; control means for connecting said pilot demodulators to be responsive to said correlators respectively during a sync acquisition mode, and connecting said data demodulators to be responsive to said correlators and connecting said adder to be responsive to the data demodulators during a tracking mode; code generator means for simultaneously and successively supplying a plurality of despreading codes of different phase positions to said correlators, respectively, to thereby cause said correlators to despread one of said spread data and pilot signals; and phase shift means for successively shifting the phase position of each of said despreading codes and determining high correlation values from output signals of said pilot demodulators during said sync acquisition mode, and setting the phase positions of said despreading codes during said tracking mode according to said high correlation values.
 2. A direct sequence spread spectrum receiver as claimed in claim 1, further comprising:a second correlator for despreading said spread data signal and said spread pilot signal; second code generator means for successively supplying a second despreading code of different phase position to said second correlator to thereby cause said second correlator to despread said spread pilot signals; and a second pilot demodulator for operating on an output signal of said second correlator during said tracking mode, said phase shift means successively shifting the phase position of said second despreading code in the neighborhood of the phase positions previously set in the sync acquisition mode and determining second high correlation values from output signals of the second pilot demodulator during said tracking mode, and updating the previously set phase positions according to the second high correlation values during an update mode.
 3. A direct sequence spread spectrum receiver as claimed in claim 1, further comprising:a second correlator for despreading said spread data signal and said spread pilot signal; a second pilot demodulator; a second data demodulator; and second code generator means for successively supplying a second despreading code of different phase position to said second correlator to thereby cause said second correlator to despread said spread pilot signals, said control means connecting said second pilot demodulator to be responsive to said second correlator during said tracking mode, connecting said second data demodulator to be responsive to said second correlator during an update mode, and introducing a ramp-up time in response to said second data demodulator being connected to said second correlator, disconnecting a successive one of the outputs of the first-mentioned data demodulators from said adder for a predetermined interval immediately following said ramp-up time, and connecting the output of the second data demodulator to said adder during said predetermined interval, said phase shift means successively shifting the phase position of said second despreading code in the neighborhood of the phase positions previously set in the sync acquisition mode and determining second high correlation values from output signals of said second pilot demodulator during said tracking mode, and updating the previously set phase positions according to the second high correlation values during said update mode.
 4. A direct sequence spread spectrum receiver as claimed in claim 1, wherein each of said data signal and said pilot signal contains in-phase and quadrature components, wherein a first group of said correlators produces in-phase despread data signals and quadrature despread data signals and a second group of said correlators produces in-phase despread pilot signals and quadrature despread pilot signals during said tracking mode, and wherein each of said data demodulators comprises:a first multiplier for multiplying the in-phase despread data signal from a first one of said first-group correlators with the in-phase despread pilot signal from a second one of said second-correlators; a second multiplier for multiplying the quadrature despread data signal from said first one of said first-group correlators with the quadrature despread pilot signal from said second one of said second-group correlators; and an adder for combining output signals of said first and second multipliers.
 5. A direct sequence spread spectrum receiver as claimed in claim 1, wherein each of said data signal and said pilot signal contains in-phase and quadrature components, wherein a first group of said correlators produces in-phase despread data signals and quadrature despread data signals and a second group of said correlators produces in-phase despread pilot signals and quadrature despread pilot signals during said tracking mode, and wherein each of said data demodulators comprises:first moving average means for successively summing a predetermined number of symbols of the in-phase despread pilot signal from a first one of the second-group correlators to produce a first moving average value; second moving average means for successively summing said predetermined number of symbols of the quadrature despread pilot signal from a second one of the second-group correlators to produce a second moving average value; first multiplier means for multiplying the in-phase data signal from a first one of said first-group correlators with the first moving average values; second multiplier means for multiplying the quadrature data signal from a second one of said first-group correlators with the second moving average value; and an adder for summing output signals of the first and second multiplier means.
 6. A direct sequence spread spectrum receiver as claimed in claim 5, further comprising:first delay means for introducing a delay to the in-phase data signal from said first one of said first-group correlators by an amount corresponding to one half of said predetermined number of symbols and applying the delayed signal to said first multiplier means; and second delay means for introducing a delay to the quadrature data signal from said second one of said first-group correlators by an amount corresponding to one half of said predetermined number of symbols and applying the delayed signal to said second multiplier means.
 7. A direct sequence spread spectrum receiver as claimed in claim 1, wherein each of said data signal and said pilot signal contains in-phase and quadrature components, wherein all of said correlators produces in-phase despread data signals and quadrature despread data signals during said sync acquisition mode, and wherein each of said pilot demodulators comprises:first integrator means for summing a predetermined number of symbols of the in-phase pilot signal from a corresponding one of said correlators to produce a first integrated signal; second integrator means for summing a predetermined number of symbols of the quadrature pilot signal from said corresponding one of said correlators to produce a second integrated signal; first squaring means for squaring the first integrated signal; second squaring means for squaring the second integrated signal; and an adder for summing output signals of said first and second squaring means.
 8. A direct sequence spread spectrum receiver as claimed in claim 1, wherein said phase shift means includes:phase position generating means for generating a plurality of sequences of phase position signals during said sync acquisition mode; a Walsh code number generator for generating a plurality of Walsh code numbers; wherein said code generator means comprises: a plurality of pseudo-random number (PN) generators for generating a plurality of PN sequences at instants respectively determined by said phase position sequences and supplying said PN sequences to said correlators, respectively; and a plurality of Walsh code generators for generating a plurality of Walsh codes determined respectively by said Walsh code numbers at instants respectively determined by said phase position sequences, and supplying the Walsh codes to said correlators, respectively, said phase shift means including control means for setting, during said sync acquisition mode, all of said Walsh code numbers to an all-zero Walsh code number so that all of said Walsh codes generated by said Walsh code generators are zero sequences causing said pilot signal to appear from said correlators, setting, during said tracking mode, a first group of said Walsh code numbers corresponding to said phase positions determined by the first-mentioned high correlation values and setting a second group of said Walsh code numbers to said all-zero Walsh code number, causing said data signal to appear from the first group of said correlators and said pilot signal to appear from the second group of said correlators, and updating, during said update mode, the Walsh code numbers of said first group according to the second high correlation values.
 9. A direct sequence spread spectrum receiver comprising:a plurality of first correlators for despreading a spread data signal and a spread pilot signal; a second correlator for despreading said spread data signal and said spread pilot signal; a plurality of first pilot demodulators; a second pilot demodulator for operating on an output signal of said second correlator during a sync acquisition mode and during a tracking mode; a plurality of data demodulators; control means for causing said first pilot demodulators to operate on output signals of said first correlators respectively during said sync acquisition mode, causing said data demodulators to operate on the output signals of said first correlators during said tracking mode and combining output signals of the data demodulators; first code generator means for simultaneously and successively supplying a plurality of first despreading codes of different phase positions to said first correlators respectively to thereby cause said first correlators to despread one of said spread data and pilot signals; second code generator means for successively supplying a second despreading code of different phase positions to said second correlator to thereby cause said second correlator to despread said spread pilot signals; and phase shift means for successively shifting the phase position of each of said first despreading codes and determining first high correlation values from output signals of said first and second pilot demodulators during said sync acquisition mode, setting the phase positions of said first despreading codes during said tracking mode according to said first high correlation values, successively shifting the phase position of said second despreading code in the neighborhood of the phase positions previously set in the sync acquisition mode and determining second high correlation values from said second pilot demodulator during said tracking mode, and updating the previously set phase positions according to the second high correlation values during an update mode. 